1. Field of the Invention
This invention relates generally to polyion-sensitive membrane electrodes, and more particularly, to a unique rotating potentiometric polyion-sensitive membrane electrode, and methods of using same, using membranes of the type having a non-equilibrium response mechanism.
2. Description of the Related Art
Recently, it has been discovered that specially formulated polymer membranes doped with appropriate lipophilic ion-exchangers yield large and reproducible potentiometric responses toward various biomedically important polyanions (e.g., heparin, DNA, and polyphosphates) and polycations (e.g., protamine and polyarginine) at μg/ml levels in the presence of physiological concentrations of common inorganic ions. Examples of such polyion-sensitive, or polyion-responsive, membrane electrodes are described in U.S. Pat. Nos. 5,453,171; 5,236,570; and 5,607,567, the disclosures of which are incorporated herein by reference.
The EMF response of these so-called polyion-sensitive membrane electrodes (PSEs) has been ascribed to the establishment of a non-equilibrium, steady-state ion-exchange process that occurs at the membrane/sample interface. This exchange occurs because of the very favorable extraction of the analyte polyion into the organic membrane phase by cooperative ion-pairing with the lipophilic ion-exchange species. A detailed description of the fundamental response mechanism of the PSEs is found in Fu, et al., Anal. Chem., Vol. 66, pages 2250-2259 (1994), the disclosure of which is incorporated herein by reference.
The polyanion-sensitive electrode devices have many useful bioanalytical applications several of which are described below. For example, accurate determinations of the level of the anticoagulant heparin in undiluted whole blood have been achieved via a simple potentiometric titration using protamine as the titrant and a polycation-sensitive membrane electrode as the end-point detector. Both polycation and polyanion-sensitive membrane electrodes have been shown to be useful as detectors for the determination of certain enzyme activities that cleave larger polyionic substrate molecules into smaller fragments of lower charge and molecular weight. Enzyme analysis applications rely on the fact that these devices exhibit much less EMF response to lower molecular weight polyions, owing to a significant decrease in the strength of cooperative ion pairing between the low molecular weight polyion and the lipophilic ion-exchanger within the membrane phase. Very recently, PSEs have been used as detectors in the development of a novel, non-separation, competitive binding immunoassay scheme in which synthetic polycationic peptides are employed as labels.
Given the large number of potential applications of PSEs, it would be desirable to further enhance the sensitivity of these electroanalytical devices. Some progress has been made in this regard. Previously, it had been discovered that lowering the plasticizer content in the polymer membrane matrix improves the sensitivity of the PSEs by reducing the diffusion coefficient of the polyion-exchanger complex. Further, it was discovered that the shape of the electrode affects the sensitivity of PSEs. A cylindrical membrane electrode design was found to be slightly more sensitive than a planar membrane configuration owing to the enhanced mass transfer of the polyion to the membrane surface by cylindrical diffusion versus planar diffusion. It was also noted earlier that stirring the sample solution, illustratively with a stir-bar, results in an improvement in analyte sensitivity for PSEs relative to the sensitivity in a non-stirred solution. Each of these observations, taken together, also helped to determine, definitively, that PSEs have an operative non-equilibrium response mechanism.
There is, however, a need for additional improvements in polyion membrane electrode sensitivity. Due to the non-equilibrium response mechanism of PSEs, improvements in sensitivity can be made by employing a method to further enhance mass transfer of the analyte polyion to the membrane/sample interface in a controllable manner. Rotating electrode voltammetry and amperometry are well-established hydrodynamic methods that yield enhanced mass transfer of analyte as a function of the rotation speed of a planar working electrode. However, rotating electrode technology has not heretofore been applied to potentiometry.
It is, therefore, an object of this invention to provide a polyion-sensitive membrane electrode with improved sensitivity.
It is another object of this invention to provide a novel polyion-sensitive membrane electrode with improved sensitivity by employing a rotating electrode to enhance mass transfer of the analyte to the membrane/sample interface.